1. Field of the Invention
The present invention relates to a camera boosting circuit.
2. Description of the Prior Art
Some cameras comprise a boosting circuit for boosting a power source voltage of 3 to 6 V up to a voltage of about 300 V for operating a flash unit. An oscillating transistor for converting DC power into AC power is generally used in this boosting circuit.
FIG. 2 shows an automatic flash unit charging circuit using a conventional camera boosting circuit. Referring to FIG. 2, reference numeral 1 denotes an oscillating transistor; 2, an oscillation stop transistor connected in the emitter-base path of the oscillating transistor 1; 3, a bias resistor; 4, an oscillating boosting circuit, constituted by a coil, a transformer, or the like, for converting a DC voltage into an AC voltage together with the oscillating transistor 1, boosting the AC voltage by the transformer, and obtaining a high DC voltage by rectifying the boosted AC voltage; and 5, a control circuit constituted by a microcomputer and the like. When an oscillation start signal (Low (L)-level signal) is output from an output terminal P.sub.1 of the control circuit 5 to a start terminal ST 4, of the oscillating boosting circuit 4, which is connected to the base of the oscillating transistor 1, is set at "L" level so as to set the oscillating transistor 1 in an operative state. As a result, oscillation is started, and an output from the oscillating boosting circuit 4 is charged in a capacitor 6. At this time, an output from an output terminal P.sub.2 of the control circuit 5 is set at "H" (High) level. When a detector 7 detects that the voltage of the capacitor 6 reaches a predetermined value, an oscillation stop signal ("H"-level signal) is output from the output terminal P.sub.1 of the control circuit 5 to the start terminal ST of the oscillating boosting circuit 4. In addition, in order to reliably stop oscillation, an "L"-level signal is output from the output terminal P.sub.2 of the control circuit 5 to the of the oscillation stop transistor 2, thus turning on the oscillation stop transistor 2. As a result, the emitter-base path of the oscillating transistor 1 is short-circuited, and oscillation is reliably stopped. Thereafter, a trigger circuit 8 is operated by a signal output from the control circuit 5, and a discharge tube 9 emits light.
In such a conventional circuit, at least two signal lines (denoted by reference symbols A and B in FIG. 2) must be connected to the control circuit 5 so as to control start and end operations of oscillation. In addition, this circuit is unstable in an oscillation stop state. That is, the circuit may be forcibly operated when, for example, it receives external strong noise or leakage occurs near the start terminal ST of the oscillating boosting circuit 4 because of dew condensation or high humidity. In order to prevent such an operation error, the oscillation stop transistor be kept ON, and the emitter-base path of the oscillating transistor 1 may be short-circuited. However, if such an arrangement is employed, the transistor 2 must be kept ON even during a period in which a flashing operation is not performed. As a result, the current consumption of the circuit is inevitably increased.